Formation of shrinkage porosity during solidification of steel: Numerical simulation and experimental validation

The phase transformations in solidification of steel are accompanied by shrinkage and sudden changes in the solubility of alloying elements, resulting in negative side effects as micro-and macrosegregation and the formation of gas and shrinkage porosities. This paper deals with the numerical and experimental simulation of the formation of shrinkage porosity during the solidification of steel. First the physical basics for the mechanism of shrinkage pore formation will be discussed. The main reason for this type of porosity is the restraint of fluid flow in the mushy zone which leads to a pressure drop. The pressure decreases from the dendrite tip to the root. When the pressure falls below a critical value, a pore can form. The second part of the paper deals with different approaches for the prediction of the formation of shrinkage porosity. The most common one according to these models is the usage of a simple criterion function, like the Niyama criterion. For the computation of the porosity criterion the thermal gradient, cooling rate and solidification rate must be known, easily to determine from numerical simulation. More complex simulation tools like ProCAST include higher sophisticated models, which allow further calculations of the shrinkage cavity. Finally, the different approaches will be applied to a benchmark laboratory experiment. The presented results deal with an ingot casting experiment under variation of taper. The dominant influence of mould taper on the formation of shrinkage porosities can both be demonstrated by the lab experiment as well as numerical simulations. These results serve for the optimization of all ingot layouts for lab castings at the Chair of Ferrous Metallurgy.